Added a warning class to button

Because changing username largely affects the application, it should
not be green, but be more of a warning change, which is why it's now
orange.

app/views/profiles/accounts/show.html.haml

@@ -57,7 +57,7 @@
         %p.light
           = user_url(@user)
         %div
-          = f.submit 'Save username', class: "btn btn-create"
+          = f.submit 'Save username', class: "btn btn-warning"
 
   - if show_profile_remove_tab?
     %fieldset.remove-account